1. Field of the Invention
The present invention relates to a terminating resistor device and a method for testing a terminating resistor circuit.
2. Description of Related Art
With a rapid advance of information technology (IT), transmission speed of data communicated on a transmission line is becoming higher and higher. To meet a variety of user needs, the type or the number of equipments that are connected to transmission lines varies, according to usage of computers or computer networks. As is known, some types of equipments that are connected to transmission lines are equipped with a variable terminating resistor device for impedance matching between a transmission line and the equipment. The equipments that are connected to transmission lines are, for example, a plurality of personal computers (PC). As for a serial or parallel interface which is used for connection between the computers, several specification are known. The computers are equipped with I/O interface circuits compliant with specification which may differ and data communication is performed between the I/O interface circuits.
Some types of I/O interface circuits are equipped with an I/O buffer in which the variable terminating resistor circuit for impedance matching is installed. By adjusting the resistance value of the terminating resistor circuit, multi-reflection of transmitted signals because of impedance mismatch between ICs for communication devices and a transmission line, distorted waveforms of the transmitted signals caused by the multi-reflection, and transmission errors involved can be suppressed. Load on a transmission line varies, depending on change in the number of equipments connected to the transmission line. By adjusting the resistance value of the terminating resistor circuit, according to such variation in the load, impedance mismatch with the transmission line can be eliminated.
Some computer in which terminating resistor devices are installed at either ends of internal bus lines is known (refer to Japanese Patent Document JP-A-Hei 10-198473). Resistors having the same resistance value as the characteristic impedance of the bus lines are used in the terminating resistor devices. The terminating resistor devices prevent signal waveform distortion which is caused by the following phenomenon. Output signals are reflected at either ends of a bus line and a reflected signal is superimposed on an output signal. Accordingly, the terminating resistor devices can prevent data transmission errors due to distorted waveforms. By user need, a variable number of IC boards are connected to the bus. By change of the IC boards connected, the bus line impedance varies. The terminating resistor devices can suppress impedance mismatch with the transmission line by changing the resistance value thereof, accommodating the IC boards connected.
As the terminating resistor devices of a variable resistance value type, which are installed at the ends of a transmission line, as described above, a terminating resistor device whose terminating resistance value is adjusted by bit switching is known. FIG. 1 is a block diagram showing a simplified structure of a terminating resistor device of a typical bit switching type. FIG. 1 gives an example of the terminating resistor device installed in a receiving circuit in an I/O buffer circuit. In FIG. 1, reference numeral 101 denotes a terminating resistor circuit which performs impedance matching with the transmission line; 109 denotes a receiving circuit which receives data carried on the transmission line; 104 denotes a decoder circuit which sends a control signal to the terminating resistor circuit; and 110 denotes terminals for bit control through which a control instruction is input to the decoder circuit. The terminating resistor circuit 101 comprises a plurality of resistor elements 102 connected in parallel and each resistor element is equipped with a control terminal. By a control signal input to the control terminal, ON/OFF control of each resistor element is performed. Each control terminal is connected to the decoder circuit 104 and the resistance value of the terminating resistor circuit 101 is determined by a control signal from the decoder circuit 104.
After manufacturing a device including the terminating resistor circuit 101, it is necessary to test that the terminating resistor circuit 101 conforms to specification. Testing the forgoing prior-art terminating resistor device is performed through the following process. Initially, from the terminals 110 for bit control, an instruction to set one resistor element ON, for example, to set a first-stage resistor element ON, is input. The decoder circuit 104 decodes the instruction and sends a control signal to the terminating resistor circuit 101. Only the first-stage resistor element of the terminating resistor circuit 101 is set ON and the remaining resistor elements are set OFF. The resistance value of the terminating resistor circuit 101 in this state is measured at input terminals 105 and 106. It is determined whether the measured resistance value conforms to the specification.
Next, an instruction to set another resistor element ON, for example, to set a second-stage resistor element ON, in addition to the first-stage resistor element, is input from the terminals for bit control 110 to the decoder circuit 104. The first-stage and second-stage resistor elements are set ON in the terminating resistor circuit 101 and the remaining resistor elements are set in the OFF state. The resistance value of the terminating resistor circuit 101 in this state is measured and it is determined whether the measured resistance value conforms to the specification. The same process is repeated for other resistor elements in order and the number of resistor elements that are set ON is incremented one by one, and the resistance value of the terminating resistor circuit 101 is measured in each step.
In this way, a conventional test method is to test whether the resistance value of the terminating resistor circuit conforms to the specification, while increasing the number of resistor element that are set ON. Therefore, the resistance value measurements of the terminating resistor circuit are susceptible to measurement errors. Moreover, because a signal for test is input via the decoder circuit to the terminating resistor circuit 101, it is difficult to discriminate between a fault in the decoder circuit 104 and a fault in the terminating resistor circuit 101.
Meanwhile, an input circuit in which an input signal waveform can be adjusted without modifying the circuit board is known (refer to Japanese Patent Document JP-A-Hei 8-162930). This circuit is configured such that a plurality of serial circuits, each consisting of a pull-up resistor and a pull-up switching element connected in series, are connected in parallel between a pull-up power supply and an input terminal, a plurality of serial circuits, each consisting of a pull-down resistor and a pull-down switching element connected in series, are connected in parallel between a pull-down power supply and the input terminal, and the circuit includes a control circuit for performing ON/OFF control of the pull-up switching elements and the pull-down switching elements